1. Field of the Invention
The present invention relates to an overspeed shutdown system for centrifuge apparatus and, more particularly, to an improved overspeed shutdown system for centrifuge apparatus which provides a sharper cutoff signal and a backup signal in the event the centrifuge rotor accelerates through the shutdown speed more rapidly than the normal shutdown circuitry can respond to.
2. Description of the Prior Art
A centrifuge apparatus generally has the capability of accepting rotors of many different speed ratings and sizes to fit the specific centrifuge application desired. Naturally, each rotor has its maximum speed rating and must be protected against overspeed and resulting damage.
Overspeed protection has been provided in the past to guard against either operator error or a malfunction of the speed control circuit. One type of overspeed protection system for centrifuge apparatus is described in my prior U.S. Pat. No. 3,436,637, issued Apr. 1, 1969. The system of such patent depends upon a beat frequency which approaches zero as the speed of the rotor approaches its maximum speed rating.
More specifically, the overspeed protection system for a centrifuge apparatus including a rotor driven by a drive motor according to my prior patent comprises a transducer for providing a signal whose frequency is a function of the rotational speed of the centrifuge rotor. A crystal controlled oscillator is provided for generating a reference signal having a predetermined frequency. A mixer responsive to the rotational speed signal and the reference signal provides a difference frequency signal. A low pass filter coupled to the output of the mixer passes signals having frequencies below a predetermined frequency. Means coupled to the filter disconnects power to the drive motor in response to an output signal from the filter.
Such an overspeed protection system inherently has some delay from the time the difference frequency signal falls below the predetermined frequency and the time that the power disconnecting means disconnects power to the drive motor. This delay results from the inclusion of the low pass filter. While this delay is of little concern in the case of large, heavy rotors which accelerate slowly, it is of great concern in the case of small, lightweight rotors which accelerate rapidly. That is, in the latter case, the rotor can accelerate past its maximum speed rating, to a point where the frequency difference between the rotational speed signal and the reference signal is greater than the predetermined frequency, faster than the power disconnecting means can respond. Since the low pass filter cannot distinguish between those cases when the rotational speed signal has a frequency greater than the frequency of the reference signal and when the rotational speed signal has a frequency below the frequency of the reference signal, there are times when an overspeed condition goes undetected with resultant damage to the centrifuge apparatus.
To prevent the above from occurring, it has been the practice heretofore to make the pass band of the low pass filter relatively wide to ensure that the power disconnecting means has adequate time to respond to the output of the filter as the beat frequency signal approaches zero and then increases away from zero. This is, however, an unacceptable solution to the problem since the result is that a shutdown signal will be generated at rotational speeds significantly lower than the maximum speed rating of the rotor. To the contrary, it is desirable to permit the rotor speed to come as close as possible to its maximum speed rating without generating an overspeed signal.